1. Field of the Invention
The present invention relates to an electric power steering apparatus in a vehicle.
2. Description of the Related Art
In an electric power steering apparatus, an electric motor generates an assist torque in accordance with a steering torque and transmits the torque to a steering shaft, and thereby reduces the steering torque. Generally, a vehicle of 2000 cc engine requires a current of 60 A or more. Therefore, a voltage higher than a battery voltage of the vehicle is required in order to gain a larger output voltage. An electric power steering apparatus which includes a step-up voltage apparatus for stepping up a battery voltage and drives the motor with a step-up voltage is disclosed. Further, the electric power steering apparatus discloses that, when an input voltage continues to be lower than a predetermined value for a predetermined time, an output voltage is gradually lowered at a predetermined inclination (as shown in JP-H08-127355).
However, a conventional step-up voltage device outputs a uniform step-up voltage (20V) when an input voltage (battery voltage) is in a range of a rated voltage (10 V to 16 V). The device reduces the output voltage at a predetermined inclination when the input voltage is equal to or lower than the range of the rated voltage. Accordingly, even a motor stops and a large mount of current flows, the step-up output voltage continues to be output until the input voltage is out of the range of the rated input voltage. Even if the input voltage is lowered, the output voltage is lowered at the predetermined inclination, which causes the current in the device to be delayed in lowering. Generally speaking, a voltage is likely to drop when a current from a worn-out battery increases. Consequently, when a current from batteries increases and a voltage drops, an electric circuit driven by the battery cannot properly operate. Further, when a large amount of current is taken out of a worn-out battery, the battery is likely to be damaged due to an internal voltage drop.